mesa: Refactor viewport setting even more

mesa: Refactor viewport setting even more

Create an internal function that just writes data into the viewport.  In
future patches this will see more use because we only want to call
dd_function_table::Viewport once after setting all of the viewport
instead of once per viewport.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>